Closed-loop pre-coding has been introduced in the Long Term Evolution (LTE) Release 8 (Rel-8) to improve the spectrum efficiency. Closed-loop pre-coding essentially requires the same set of pre-coding matrixes, referred to as a codebook, to be stored at both an evolved Node B (eNB) and a User Equipment (UE). The UE estimates channel information from a cell common pilot and then selects a pre-coding matrix from the codebook according to some criterion which can be the maximizing mutual information, the maximizing output signal to interference and noise ratio, etc. The UE feeds back an index of the selected pre-coding matrix in the codebook to the eNB over an uplink channel, and the index is referred to as a Pre-coding Matrix indicator (PMI). The eNB can determine, from the value of the received index, the pre-coding matrix to be used for the UE. The pre-coding matrix reported by the UE can be considered as a quantized value of channel state information.
In an existing cellular system, an array of antennas of an eNB is typically arranged horizontally as illustrated in FIG. 1 and FIG. 2. A beam at a transmitting end, the eNB, can be adjusted only horizontally with a common vertical down tilt angle for every UE, so various beam-forming/pre-coding technologies are generally applied based upon channel information in horizontal dimension. In fact, a radio signal propagates in three dimensions in space, and the performance of the system may not be optimized with this common vertical down tilt angle. Adjusting of the beam in vertical direction may be of great significance to the improved performance of the system. Along with the development of antenna technologies, an array of active antennas with each array element being separately controllable has emerged in the industry as illustrated in FIG. 3A and FIG. 3B. Dynamic adjusting of a beam in vertical dimension becomes possible with this array of antennas. 3D beam-forming/pre-coding may be performed in a Frequency Division Duplex (FDD) system based upon channel state information reported by a UE, possibly using a codebook as conventionally used in the LTE Rel-8 system. However the existing codebook is designed for horizontal beam-forming/pre-coding, so a direct application thereof to 3D beam-forming/pre-coding may degrade the performance.
In summary, the existing codebook is designed for horizontal beam-forming/pre-coding, so a direct application thereof to 3D beam-forming/pre-coding may degrade the performance.